In the attempt to optimize the efficiency of turbomachines, e.g. compressors, gas turbines, steam turbines, any processes resulting in losses are minimized as far as possible. Losses are caused by leakage flows which pass through the gap between a rotating component and a stationary component of the turbomachine. In order to reduce the leakage flow, the clearance between these components has to be set to a minimum during operation.
From several reasons mutual contacts between the surfaces of the rotating and stationary components occur during operation with the consequence of surface abrasion and thus increasing gap and leakage flow.
There are different methods to validate clearance prediction in a turbomachine. One way of validation is to assess the rubbing that occurs during operation by measuring the rub at the components, i.e. measuring the casing rub and the blade wear or the shaft rub and the vane wear.
The increase in clearance between the blade tip and the stator comprises the casing rub and the blade wear. Considering the machine efficiency a rub at the stationary component is generally preferred compared to the wear of the rotating component. E.g. the casing rub leads to a local increase in clearance, whereas the wear at the blade tip leads to a clearance increase all around the section.
Measuring the blade wear gives a feedback on the rub proportion that occurs at the blades.
Known solutions for monitoring the wear at a component of a turbomachine without removal of this component from the machine comprise to provide the relevant surface of said component with a marker. At least one, preferably a number of hollow markings of a determined depth is machined into the relevant surface of the component to be monitored. Endoscopic examinations are carried out successively to provide an image signal of the markings.
Such a method for monitoring wear at compressor blades using markers is disclosed in WO 2013/050688.
It is a disadvantage that this method requires a modification of the individual component, such as blades or vanes.
According to another method, disclosed in WO 2009/085430, the height of a gap between a blade tip and the inner surface of the surrounding casing in a gas turbine engine is measured by a measuring tool. This measuring tool essentially consists of a frame, comprising two components: a backing portion and an extended portion. The backing portion includes an attachment portion, designed to install the tool on the fan case of the gas turbine engine. For this purpose measuring tool and fan case are equipped with respective fastening means, e.g. a flange at the fan case has holes for receiving screws from the backing portion of the frame. And the extended portion consists of or holds a supporting arm for a sensor. One end of the supporting arm is attached to the backing portion and its other end is free and holds the thickness measuring sensor. The length and width of the supporting arm are designed to protrude into the gap between the blade tip and the inner surface of the casing. The sensor is a non-contact sensor, such as an optical or a capacitive position sensor. After the measuring tool has been locked in its position at the engine, the fan blades are rotated and the sensor may obtain data of each of the rotating blades. By lengthwise adjustment of the supporting arm tip clearance measurements in different longitudinal positions can be made.